Extragalactic content of gamma-ray burst localizations

Results of a deep, near-infrared survey of gamma-ray burst localizations are presented. This systematic study utilized a predetermined, consistent sample of bursts and is the first to be designed specifically to examine the extragalactic content of these regions of sky. Multi-wavelength images of nine of the smallest error boxes from the third Interplanetary Network and 70 arcmin{dollar}sp2{dollar} of control fields were obtained. Capitalizing on recent advances in infrared detector technology, these images reach levels up to 200 times fainter than previous infrared studies of burst locations. Photometric measurements and star/galaxy separation of more than three thousand detected objects were made.; Suitable host galaxy candidates are found within each error box, which apparently contradicts the results of previous surveys. The previous surveys are reviewed and found to be consistent with the presence of suitable hosts. Potential problems with the original tests for the presence of potential host galaxies are identified, and a new method is proposed.; An overabundance of brighter galaxies exists in the gamma-ray burst images at the 98% confidence level. This enhancement is seen both within the error boxes and in the sky immediately surrounding them, and it indicates that bright gamma-ray bursts are cosmological in origin. Based upon their magnitudes and colors, these galaxies appear to have distances {dollar}(zsim0.2){dollar} and luminosities {dollar}(Lsim L*){dollar} consistent with gamma-ray burst hosts. As a group, the brightest galaxy within each error box is slightly brighter than normal but still consistent with a random population of galaxies. Their positions within the boxes are also consistent with a random distribution, with no evidence of clustering toward the center. The bright galaxy excess in these regions of sky, combined with a seemingly random population of brightest error box galaxies, suggests that either the true host galaxies are fainter than this, or the error boxes do not point accurately to the host galaxies.